1. Field of the Invention
The present invention relates to a battery control apparatus for controlling a battery mounted in a hybrid vehicle, more particularly to a battery control apparatus for controlling the amount of charging/discharging in accordance with the remaining charge of the battery using a nickel compound.
This application is based on Japanese Patent Application No. Hei 11-246744 (Unpublished), the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, hybrid vehicles equipped with a motor in addition to an engine as a power source for driving are known. Hybrid vehicles include series hybrid vehicles and parallel hybrid vehicles. In series hybrid vehicles, a motor is driven by the generated output of a generator driven by an engine, and the wheels are driven by the motor. As described above, the engine and the wheels are not mechanically coupled to each other in series hybrid vehicles. Therefore, the engine can be operated at an almost constant RPM reducing fuel consumption and emissions, to provide lower fuel consumption and emissions than can be achieved by conventional engine vehicles.
As opposed to series hybrid vehicles, in parallel hybrid vehicles, a motor coupled to an engine supplements the driving of the driving shaft by the engine, and electric energy obtained by using the motor as a generator is stored in a power storage unit. In addition, the generated electric energy is used for electrical equipment installed in the vehicles. As described above, since the parallel hybrid vehicle can also reduce the drive load of the engine, lower fuel consumption and emissions, compared to conventional engine vehicles, can also be achieved.
The parallel hybrid vehicle includes a type in which the motor which supplements the output of the engine is directly coupled to the output shaft of the engine, and the motor functions as a generator during deceleration and stores the generated electricity in a battery, and a type in which driving force is generated by use of both, or one of, an engine and a motor, and a generator is provided separately.
In such hybrid vehicles, various kinds of controls are performed in such manner that the motor supplements the output of the engine during acceleration, and electricity is stored in the battery by deceleration regeneration during deceleration. This makes it possible to respond to the requirements of the driver while providing electrical energy to the battery (hereinafter referred to as "remaining battery charge" or "state of charge").
The batteries used in hybrid vehicles often use nickel compounds in their positive electrode. In batteries using a nickel compound, a phenomenon in which an electromotive force value for the remaining battery charges changes due to repeated cycles of charging and discharging occurs. This phenomenon is referred to as a "memory effect" below. The memory effect has the characteristic that it is apt to occur in a battery using a nickel compound when the battery is repeatedly recharged before it has reached a completely discharged state, or repeatedly discharged before it has reached a completely charged state.
The batteries used in hybrid vehicles are controlled so as to be charged and discharged in the middle range of their remaining charge, to provide a long lifetime and high efficiency. For this reason, the batteries never reach the completely discharged state or the completely charged state, and they tend to develop a memory effect.
Since a range of the remaining battery charge available is narrowed by the occurrence of the memory effect, there is the problem that the time during which an operation for continuously supplementing the driving force of the engine and an operation for charging the battery by a deceleration regeneration can be continuously performed is reduced. Moreover, if the charge/discharge is performed only in a middle area of the remaining battery charge in a state in which the memory effect has occurred, the memory effect will further occur, and the range of the remaining battery charge available is reduced, resulting in a vicious cycle.